RFR 8204947: Port ShenandoahTaskTerminator to mainline and make it default

[Roman Kennke]

I believe if you add a flag, you need to file a CSR.

Will have a deeper look at the patch later..

Thanks,
Roman


> Hi,
> 
> ShenandoahTaskTerminator is based on Google's work stealing protocol,
> described in paper [1]. The implementation diverges in a way to
> emphasize termination latency vs. power efficiency (see comments in code
> for details).
> 
> Shenandoah switched to ShenandoahTaskTerminator as default terminator,
> shortly after it was implemented, and has been very stable since.
> 
> On performance side, it showed better termination latency on almost all
> tests/benchmarks I ran, comparing to ParallelTaskTerminator. As the
> result, we (Shenandoah) purged fallback branches and uses it as the sole
> task terminator.
> 
> This patch introduces a new experimental flag "UseOWSTTaskTerminator"
> (default: true), and switches G1 and Parallel GC to use it as default
> terminator. You can switch back to ParallelTaskTerminator with JVM
> option "-XX:+UnlockExperimentalVMOptions -XX:-UseOWSTTaskTerminator"
> 
> I did not port it to CMS, as it is depreciated.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8204947
> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8204947/webrev.00/
> 
> Test:
>   hotspot_gc and vmTestbase_vm_gc on Linux 64.
> 
> 
> [1] Understanding and improving JVM GC work stealing at the data center
> scale (http://dl.acm.org/citation.cfm?id=2926706)
> 
> Thanks,
> 
> -Zhengyu

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